1. Field of the Invention
This invention relates in general to a timing recovery device for a precoded communication system, and more particularly to a synchronization system that minimizes transient sampling fluctuations upon transition between modes of communication.
2. Description of Related Art
The telephone networks currently in place were originally designed for transmission of electrical signals carrying human speech. Since human speech is generally confined within a band ranging from 0 Hertz to 3,400 Hertz, telephone networks were designed to provide telephone lines to each user which were capable of handling frequencies within this range. Today, these same telephone lines, which connect a service user to a central office, are in place, permitting communication of only voice data or analog modem transmissions of not more than 56,000 bits per second. However, connections between central offices of telephone networks are provided by high-bandwidth fiber optic transmission facilities in nearly every telephone network worldwide.
Because the local telephone lines which connect an end user to a central office are only capable of handling frequencies of up to 3,400 Hertz, communication equipment utilizing these lines, such as dial modems or fax modems, have been accordingly limited in bandwidth. Despite the presence of high bandwidth fiber optic lines between central offices, users remain limited in the bandwidth available to them because the local lines serve as a bottleneck. New technologies, such as the Internet or video conferencing, demand that the bottleneck be removed.
Digital Subscriber Line (DSL) technologies are capable of removing the bottleneck. DSL permits a user to communicate over the existing telephone lines at a rate of tens of millions of bits per second. In order to utilize DSL, a site must be equipped with a transceiver (a DSL modem) which communicates, via the existing telephone lines, with another transceiver located at the central office of the network access provider, generally the local telephone company.
As already stated, DSL transceivers are designed to operate at high data rates. One difficulty encountered by DSL transceivers is intersymbol interference. Intersymbol interference is the superposition of a residual portion of a previous waveform over a present waveform. Such an event causes the present waveform to be increased in amplitude by the residual portion of the previous waveform, and may cause the present waveform to be misinterpreted by the receiving transceiver. Intersymbol interference is due, in part, to the high data rate of DSL transceivers.
One means of dealing with intersymbol interference is to implement a system of preceding. In general, precoding refers to various techniques of distorting a signal to be transmitted in order counteract the anticipated distortional effects of the medium across which the signal is to travel. Before a signal may be precoded, the medium across which it will propagate must be characterized. In the case of DSL transceivers that implement precoding, the local telephone lines connecting the service user and the network access provider must be characterized.
During the characterization process, DSL transceivers communicate in a non-precoded mode. In other words, until the characteristics of the medium are determined, the waveforms to be transmitted cannot be pre-distorted in anticipation of the effects of the medium.
In either precoded or non-precoded mode, communication between DSL transceivers is synchronous. Synchronization between transceivers is accomplished by a timing recovery circuit that adjusts the transceiver""s sampling based on the pulse shape of the data to be received. Upon a transition from non-precoded to precoded communication (which occurs after the medium has been characterized), the timing pulse may suddenly change shape, as it will henceforth be pre-distorted by the transmitting transceiver. This change in shape can have a disruptive influence on the timing recovery circuit, causing both a transient in timing and a permanent phase movement. The permanent phase movement can cause a performance degradation because the precoded system was trained on the previous phase.
It can be seen that there is a need for an invention which minimizes the chance of losing synchronization when switching from non-precoded to precoded mode.
It can also be seen that there is a need for an invention which minimizes or eliminates both the transient effect and the permanent timing phase movement that results from a sudden change in shape of the timing pulse.
To overcome the limitations in the prior art described above, and to overcome other limitations that will become apparent upon reading and understanding the present specifications, the present invention discloses a synchronization system that minimizes transient and permanent sampling fluctuations upon transition between non-precoded communication to precoded communication.
The present invention solves the above-described problems by providing a system that minimizes the transient effect and permanent phase movement associated with a transition between modes of communication. Such a system minimizes the chance of losing synchronization when switching from a non-precoded mode to a precoded mode of communication, and prevents a phase movement from causing a degradation in performance in precoder mode.
A system in accordance with the principles of the present invention includes a transceiver unit with a digital front end, the sampling of which is controlled by a timing recovery system. Such a timing recovery system alters the sampling rate of the front end of the transceiver based upon a process of minimizing the difference between an estimate of the current phase of the received signal and a phase reference. Upon transition between communication modes, an adaptation period is initiated, during which the sampling rate is held constant. Upon expiration of the adaptation period, the phase reference is changed to be an estimate of the phase of the received signal just prior to expiration of the adaptation period.
Other embodiments of a system in accordance with the principles of the invention may include alternative or optional additional aspects. One such aspect of the present invention is that the timing recovery system may be constructed of a phase reference selector and a timing control system. The phase reference selector captures the phase estimate immediately prior to termination of the adaptation period, outputting a predetermined phase reference until such termination, at which point the stored phase estimate is outputted. The timing control system minimizes the difference between the phase estimate and the output of the phase reference selector by altering the sampling rate, except during the adaptation period, during which the sampling rate is held constant.
Another aspect of the present invention is that the phase reference selector may be constructed of a multiplexer and a register. The register captures the phase estimate immediately prior to termination of the adaptation period. The multiplexer selects, as its output, a first predetermined phase reference value until the expiration of the adaptation period, at which time the value in the register is selected.
Another aspect of the present invention is that the timing control system may be constructed from a feedback circuit and an adder. The adder produces an error signal by calculating the difference between the selected phase reference and the current phase estimate. The feedback circuit minimizes the error signal by adjusting the sampling rate, except during the adaptation period when the sampling rate is held constant.
These and various other advantages and features of novelty which characterize the invention are pointed out with particularity in the claims annexed hereto and form a part hereof. However, for a better understanding of the invention, its advantages, and the objects attained by its use, reference should be made to the drawings which form a further part hereof, and to the accompanying descriptive matter, in which there are illustrated and described specific examples of an apparatus in accordance with the invention.